BTW, is there any difference in light conditions between APS-C lenses and full 35mm lenses? Of course 35mm gets more light to the larger area, but if using a full 35mm lens on an APS-C camera I think that all this extra light (comming thru the borders) is unuseful, right? The full 35mm are hevier and probably will give less vignetting, but ill give more light to the actual subject?

I actually use mostly Full Frame lenses on my T1i, I had them form my eos 1. And they perform quite well, once I got uses to the crop factor thing. I uses to use the 50mm prime allot on full frame and not to much a 28mm prime. But since moving to the T1i, I find myself using a 28mm prime way more. So I recently traded my ef 28mm 2.8 for a used ef 28mm 1.8 USM lens instead. And I am loving it.

If I went for a sigma 12-24mm wide angle that works on both FF and crop, I was told I would see the vignetting on the ff not the crop body at the wider end.